From ec6e6abae7629800fec4c715957c4932946f51ed Mon Sep 17 00:00:00 2001
From: Jeremy Attali <jeremy.attali@gmail.com>
Date: Sun, 24 May 2020 14:33:50 -0400
Subject: [PATCH] fix(render): fix arrow glitch with 0 ftx
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The use of `acos(θ)` works better than `atan(θ)`. Just need to be
careful with the `sign` of `fty`.
---
 src/render.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/src/render.c b/src/render.c
index c7ce44a..1c8a4c0 100644
--- a/src/render.c
+++ b/src/render.c
@@ -1,5 +1,6 @@
 #include <gdk/gdk.h>
 #include <gtk/gtk.h>
+#include <math.h>
 #include <pango/pangocairo.h>
 
 #include "swappy.h"
@@ -105,11 +106,7 @@ static void render_shape_arrow(cairo_t *cr, struct swappy_paint_shape shape) {
     return;
   }
 
-  double theta = atan(fty / ftx);
-
-  if (ftx < DBL_EPSILON) {
-    theta = M_PI + theta;
-  }
+  double theta = copysign(1.0, fty) * acos(ftx / ftn);
 
   // Draw line
   cairo_save(cr);